1. Field of the Invention
The present invention relates to a power supply circuit and, in particular, to a power supply circuit applicable to a driving circuit of a display device.
2. Description of Related Art
There is a growing demand for larger screens and higher resolutions of display devices built into mobile apparatuses such as mobile phones and PDAs. As the screens of a display device become larger, the data lines on the display panels become longer. Accordingly, the parasitic capacitances of the data lines on the display panels increases. As the resolutions of display devices become higher, the number of pixel control switches connected to the data lines on the display panels increases. With the increase in resolution of the display devices, the number of data lines increases. Accordingly, the total value of parasitic capacitances of display panels increases.
To properly drive a display panel having parasitic capacitances, a driving circuit having a high output-current-supply capacity is required. MOS transistors are generally used as driving circuits for display devices. In such a driving circuit, an increase of the output-current-supply capacity means an increase in the parasitic capacitance of the whole driving circuit connected to a power supply line.
When a load with a large capacitance is driven, an in-rush current can flow in the load at power-on. The larger the capacitance viewed from the power supply and the smaller the resistance in the path to the capacitance, the greater the in-rush current is. When an in-rush current flows, a counter-electromotive voltage expressed by the following Equation (1) can be generated:E=−L·di/dt  (1)Here, E represents the counter-electromotive voltage (V), L represents the value of inductance (H) viewed from the power supply, and i represents the power supply current (A). As can be seen from Equation (1), the counter-electromotive voltage is a generated voltage opposite in polarity to the power supply. Accordingly, a large value of the counter-electromotive voltage can cause a failure in the power supply itself or a device that is a load of the power supply.
Furthermore, an in-rush current can reduce the life of the wiring. Typically, an in-rush current has a current value several- to several-tens-fold greater than a normal operating current. Therefore, in the case of a device that is frequently turned on, the in-rush current value is more likely to affect the wiring life of the power supply line. In a device such as a display device for a mobile apparatus in which the power supply line is provided on a semiconductor device, the wiring film thickness of the power wiring is thin. Therefore, the influence of in-rush current values in the display devices built in mobile apparatuses on the wiring life has been a great concern.
In general, mobile apparatuses use a battery (rechargeable battery) as the main power supply of their systems. The battery supplies a constant voltage. Therefore, a semiconductor integrated circuit including a driving circuit for a display device for a mobile apparatus has a step-up power supply circuit in its semiconductor device for internally generating multiple voltages required for the display device. The step-up power supply circuit is a circuit that generates a boosted power supply higher than the voltage of a main power supply or a step-down power supply generating a negative voltage (hereinafter step-down is also referred to as “step-up” because step-down is a kind of step-up). An example is a charge-pump step-up power-supply circuit that changes the connection of a step-up capacitor charged with an input voltage to a series connection with the input voltage to accomplish voltage step-up by time-division driving.
The charge-pump step-up power-supply circuit requires charging the step-up capacitor in a short time in order to create a voltage source required for time-division driving in which recharge and discharge are repeated. Therefore, the step-up capacitor is connected to an input power supply by using a low resistance. In order to provide a current value of an entire driving circuit that operates at the voltage by time-division driving with a voltage drop in a specified range, the step-up capacitor needs to have a sufficiently large capacitance with respect to the current value in normal operation of the load. That is, the charge-pump step-up circuit is a large capacitance connected using a low resistance when viewed from the power supply, and can increase an in-rush current at power-on.
Furthermore, low power consumption is essential for mobile display devices in order to increase battery life and therefore the display device is frequently turned on and off. This increases the frequency of occurrence of in-rush current. Techniques for suppressing in-rush currents are known (for example, see Patent Documents 1 and 2).
FIG. 1 is a block diagram showing a technique described in Patent Document 1 (Japanese Patent Application Laid-Open No. 2002-116828). As shown in FIG. 1, in a circuit described in Patent Document 1, an in-rush current suppressing transistor 111 is connected in series with a power-supply path. The in-rush current suppressing transistor 111 controls a power supply current. The circuit described in Patent Document 1 is configured so that its resistance reaches the maximum at power-on and becomes minimum when it is determined that the in-rush current has disappeared. FIG. 2 shows waveform charts illustrating operation of the circuit described in Patent Document 1. FIG. 2A shows the gate-source voltage of the in-rush current suppressing transistor 111. FIG. 2B shows the drain-source voltage of the in-rush current suppressing transistor 111. FIG. 2C shows an input current of a DC/DC converter.
As shown in FIG. 2A, the rate of increase of a power supply current at power-on is reduced to a very small value and the power supply current is gradually increased and, at the time point when it is determined that the in-rush current has disappeared, that is, the voltage supplied to the load approaches a specified value, the power supply current value is rapidly reduced back to a normal current value in the circuit in Patent Document 1. The circuit described in Patent Document 1 operates as shown in the waveform to minimize the initial counter-electromotive voltage.
FIG. 3 shows waveform charts illustrating operation described in Patent Document 2 (Japanese Patent Application Laid-Open No. 2002-091584). As shown in FIG. 3, a technique described in Patent Document 2 constantly increases a power supply current Iout from the value at power-on and, when an output voltage approaches a specified value, the power supply current is rapidly reduced back to a normal power supply current value. The circuit operates as shown in the waveform to suppress the counter-electromotive voltage to a fixed value.